The present invention relates to a composition of matter containing the uranyl ion. More specifically, the present invention relates to such compositions of matter which are useful in uranium isotope separation processes.
The significance of the present invention resides primarily in the fact that the discovery of such volatile uranyl compounds permits their use for the separation of isotopes, as is disclosed in copending application Ser. No. 865,963, filed on Dec. 30, 1977, and assigned to Exxon Research and Engineering Company, the assignee of the present application. The compounds of the present application are volatile uranyl ion containing compounds which have an isotopically shifted infrared absorption spectrum associated therewith, and which therefore can be used to separate isotopes by vaporizing the subject compound and irradiating same with infrared radiation which is preferentially absorbed by a molecular vibration of molecules therein containing a predetermined isotope of the element to be separated. This provides excited molecules of this compound enriched in the molecules of the compound containing that predetermined isotope, thus enabling separation of those excited molecules. In addition, these particular molecules exhibit a number of properties which render them particularly useful in such processes, such as an isotopic shift within the range of wavelengths of from about 810 to 1116 cm.sup.-1, i.e. within the range of commercially available CO.sub.2 lasers, and they exhibit relatively high vapor pressures at relatively low temperatures.
U.S. Pat. No. 3,951,768, which was issued on Apr. 8, 1976 to Carl Gurs on an application filed before the effective date hereof, discusses the use of a CO.sub.2 laser for the separation of isotopes, and mentions as a specific compound therein UO.sub.2 (NO.sub.3).sub.2.6H.sub.2 O along with a number of other compounds as possibly being useful for the separation of uranium isotopes. This patent thus appears to suggest the use of uranyl compounds with a CO.sub.2 laser for isotope separation, however it should be noted that since the compound listed in the Gurs' patent is listed along with others which do not readily absorb light from the CO.sub.2 laser, it is not clear what was exactly intended to be taught therein. Nevertheless, a uranyl compound is mentioned. The specific compound in question, however, is believed to be non-volatile in the sense that it decomposes and therefore cannot be employed in the vapor phase for isotope separation. In fact, most uranyl compounds decompose without vaporization when heated, and it is this fact which renders the invention of the compounds of the present invention, which possess such volatility, of such significance.
With further regard to the compounds of the present invention, early reports on uranyl-containing compounds were made by Messrs. Schlessinger and Brown in the late 1940's. Thus, in U.S. patent application Ser. No. 662,600 published in the Official Gazette on Mar. 6, 1951, Chemical Abstract 46, 10192b, those authors disclose a class of uranyl-containing .beta.-diketone compounds which they investigated in connection with vapor phase processes for gas diffusion and uranium ore separation. They thus disclose compounds having a general formula as follows: ##STR1## where R may be a fluoro substituted alkyl group and R.sub.1 a halogen substituted radical. Subsequently, however, these same individuals, in the Journal of the American Chemical Society, 75, pages 2446-8 (1953) went on to report that "there is little likelihood of finding such compounds having vapor tensions above 0.1 mm. at 130.degree.." In that article the only vapor pressure reported by the authors for a .beta.-diketone (in this case for UO.sub.2 (1,1,1-trifluoroacetylacetone).sub.2 was 0.0027 torr at 130.degree. C.
These articles, in addition to several other articles, do discuss the relationship between increased volatility and fluorination. Messrs. Schlessinger and Brown, for example, discuss the increase in volatility achieved by replacement of the methyl radicals of acetylacetone by the trifluoromethyl group. They conclude, however, that based on their observations the search for a significantly more volatile uranium compound of the diketone type "held little promise of success."
One uranyl ion-containing compound which has been reported, although sparsely, is uranyl hexafluoroacetylacetonate, ##STR2## UO.sub.2 (hfacac).sub.2 and its complexes. Belford et al (J. Inorg. Nucl. Chem. 14, 169 (1960) first prepared and described a "tetrahydrate" which presumably (no formula is given in the reference) is UO.sub.2 (hfacac).sub.2.4H.sub.2 O, and which decomposes above 58.degree. C. In this article the authors show the infrared absorption bands for various uranyl compounds, and discuss the effect of ligand substitution on the visible spectra. The authors thus conclude that the more basic ligands attach more securely to the uranium atom, decreasing its coordinating tendency.
Hfacac, which is a chelating anion, is known to stabilize metal salts and allow for volatile species (Kutal, J. Chem. Ed. 52. 319 (1975)). Furthermore, this anion has no bands in the infrared region of 900-1000 cm.sup.-1, the region where the UO.sub.2.sup.+2 group has a strong antisymmetric stretching mode, which is of interest for any isotope selective CO.sub.2 laser irradiation.
In general, uranyl compounds can have five or six atoms coordinated to the central U ion (in addition to the oxygens of the uranyl), but preferably five such atoms. Where the hfacac anion is utilized, each hfacac group uses two coordination sites, and thus in the case of the preferred configuration with five atoms coordinated to the central U atom, this leaves one open site for a neutral ligand which is necessary to produce a stable uranyl containing vapor. In the absence of an appropriate stabilizing neutral ligand it is impossible in this case to generate the stable monomeric vapor of the uranyl (hfacac).sub.2. The Belford et al paper described above shows that water molecules are not suitable as neutral ligands since the compound does not volatilize intact so as to form such a stable vapor phase species.
If no other molecules are present, UO.sub.2 (hfacac).sub.2 will dimerize, sharing two of the oxygens, and thus each UO.sub.2.sup.+2 ion will have the requisite five oxygens around it. This dimer is not desired for laser isotope separations (monomers are preferred) because its volatility is too low and it allows for scrambling of absorbed energy and reduction in selectivity. Energy absorbed by the selectively excited UO.sub.2.sup.+2 group is diluted due to enhanced transfer to the second UO.sub.2.sup.+2 group which is intimately bound to it in a dimer or oligomer. Therefore, judicious selection must be made to choose a neutral Lewis base molecule to stabilize the uranyl (hfacac).sub.2 as a monomer, to confer high volatility, and to have no infrared bands which might interfere with the UO.sub.2.sup.+2 asbsorption. In addition, the presence of this neutral ligand may also enhance any subsequent photochemical reaction desirable in an isotopic selective process.
There have been reports of UO.sub.2 (hfacac).sub.2.L compounds in which L=aromatic amine oxides (Subramanian et al, Journal Inorg. Nucl. Chem. 33, 3001 (1971)), phosphine oxides and sulfoxides (Sieck, Gas Chromatography of Mixed-Ligand Complexes of the Lanthanides and Related Elements, Ph.D. Thesis, Iowa State Univ., 1971)). However, while the Subramanian et al article discusses compounds such as pyridine N-oxide as such a ligand, there is no discussion of the volatility of such compounds, and the article further indicates that the amine N-oxides were selected by the authors over such materials as alcohols, ethers and amines because of the more polar nature of these compounds and the search for a stronger bond therewith. In addition, the Sieck thesis, as well as two other articles by Sieck in Chemical Abstracts, 75, 147395Q and at Nuclear Science Abstracts, 25 (17), 39410 (1971), include discussions regarding the use of these mixed ligand complexes for the separation and detection of UO.sub.2.sup.+2, and mention is made of the detection of these complexes by gas chromatography at column temperatures of about 200.degree. C.
Mitchell (Synergic Solvent Extraction and Thermal Studies of Fluorinated Beta-Diketone-Organophosphorus Adduct Complexes of Lanthanide and Related Elements, Ph.D. Thesis Iowa State Univ., 1970) prepared the tributylphosphate complex of UO.sub.2 (hfacac).sub.2 and showed that it sublimed at about 150.degree. C. But, as discussed above, significant vapor pressure at lower temperatures is desired in an isotope separation process and a sublimation temperature below about 130.degree. C. or even near or less than 100.degree. C. is much preferred.
Very recently a comprehensive review of the complex chemistry of a number of uranyl compounds with various chelating ligands appeared in which the paucity of work on UO.sub.2 (hfacac).sub.2 and its complexes was evident (Casellato et al, Inorg. Chimica Acta, 18, 77 (1976)). The authors review the behavior of the actinides when complexed with various organic chelating ligands, such as the .beta.-diketones. This article again indicates the contribution that fluorination plays in the volatility of these compounds, and in particular with regard to complexes of the type UO.sub.2 (acetylacetonate).sub.2 L. It is at this point in that article that it is indicated that the monodentate ligands (L) begin to come off between 83.degree. and 170.degree. C., followed by decomposition of the complex, even though ligands containing nitrogen donor atoms are said to result in decomposition temperatures that are much higher. Again, no direct discussion of volatility is contained in this portion of the Casellato et al article, which goes on to discuss the relationship between ligand selection and shifts in the absorption spectra of the molecule.
The effects of chelate and ligand substitution on the IR spectra is also discussed by Haigh and Thornton in "Ligand Substitution Effects in Uranyl .beta.-ketoenolates," Chemical Abstracts 75, 55935n, and a further discussion of the effect of fluorine substitution for hydrogen on volatility is made by Swain et al in "Volatile Chelates of Quadrivalent Actinides." Inorganic Chemistry, Vol. 9, No. 7, Pages 1766-9 (1970) which relates to tetravalent uranium compounds, the most volatile of which is U(CH.sub.3 COCHCOCF.sub.3).sub.4, a compound which the author states to be ". . . too unstable for any practical use."
There is also an article by Bloor et al (Canadian Journal of Chemistry, 42, 2201-2208) which teaches the existance of a compound described as uranyl phthalocyanine, which is said to be sublimable under a vacuum "below 0.01 mm pressure at 400.degree.-450.degree. C."
Finally, in a series of articles by Levy et al and Taylor et al, J. C. S. Dalton, 1628-1640 (1977), the authors discuss their studies of UO.sub.2 (hfacac).sub.2 trimethylphosphate. In particular, they studied the thermal effects upon this complex, and concluded that their findings concerning the crystal structures of this compound indicates that polymorphism occurs on heating. It is noted that the boiling point of trimethylphosphate is greater than 190.degree. C. More significantly, however, these articles include no discussion of the specific volatilies or stabilities of these complexes, and these articles do not teach that they or any other related types of complexes would possess the properties of the compounds of the present invention. In fact, quite unexpectedly in view of the disclosures in this series of articles, the applicants have discovered that the trimethylphosphate compound disclosed therein has a higher vapor pressure and volatility than one would expect therefrom, and furthermore that it would be a useful compound in processes such as those discussed herein and disclosed in co-pending application Ser. No. 865,963.
None of these articles thus teach the specific uranyl compounds which are claimed as constituting the present invention. Beyond their mere failure to teach or suggest these compounds, however, stands the fact that there references demonstrate a state of the art in which no uranyl compounds of this nature are shown which have significant volatilities at relatively low temperatures. Lack of any teaching of such compounds, when coupled with the desire to attain uranyl-containing compounds having such properties, is significant proof of the patentable nature of the particular compounds invented by the applicants. It is in this regard that reference is made to co-pending application Ser. No. 865,963, which application claims the use of the present compounds in a process for the separation of isotopes therewith.
There are, however, other uses for these compounds which also require that they be volatile uranyl compounds, such as in the separation of heavy metals by either gas chromatography or fractional sublimation. Such separations are important in mineral treatments and in reprocessing spent nuclear fuels, etc., in which uranium (present as uranyl ion) is to be separated from other rare earth metal ions, actinide metal ions, or other metal ions.